This invention relates to (1) a method for producing attrition-resistant particles of dolomite or limestone, (2) the resulting composition, and (3) to an adsorption process for removing sulfur dioxide from a gas using said particles as the adsorbent. More particularly, it relates (1) to a method for partially coating said particles with a glaze-forming flux by heating a mixture of said particles and flux at a suitable temperature; (2) to the resulting composition, and (3) to a process for removing sulfur dioxide from a combustion gas.
U.S. Pat. No. 2,718,453 teaches a process for removing sulfur dioxide from combustion gas by contacting the gas with finely divided oxides, hydroxides, and carbonates of metals of the alkaline earth series at a temperature of at least 350.degree. F. This process is disadvantageous in that (1) the adsorbents are, in general, expensive relative to natural minerals such as dolomite, limestone and the like, and (2) these materials, as well as limestone or dolomite, are undesirably attrited under gas-solid contacting conditions normally employed in adsorbing an impurity, for example sulfur dioxide, from a gas. As a result ultra finely divided particles of the adsorbent and/or products associated with spent adsorbent are produced. To avoid atmospheric pollution and in order to meet air quality standards, particulates must be removed from the gas prior to its release to the atmosphere.
An object herein is to provide a sulfur dioxide adsorbent having improved attrition resistance, as well as an improved process using these particles to remove sulfur dioxide from a combustion gas produced by burning a sulfur-containing fuel.
A method is provided for improving the attrition resistance of a particulate solid selected from limestone and dolomite comprising the steps of:
(a) forming a mixture of the particulate solid with a finely divided solid material including at least one glaze-forming flux having a melting point lower than the particulate solid selected from phosphates, silicates and borates of alkali and alkaline earth metals and oxides of boron, the mixture containing from 0.002 to 1 part, by weight, of the finely divided solid per 100 parts of the particulate solid; and
(b) glazing a fraction of the exterior of the particulate solid with the finely divided solid by maintaining the mixture at a temperature below the melting temperature of the particulate solid and high enough to melt the flux of about 0.1 to 30 minutes in a fluidized bed.
Another aspect of the invention is the attrition resistant composition resulting from the above method.
In a yet further aspect of the invention, sulfur dioxide is removed from a combustion gas by contacting said gas with a bed of particles appropriately sized for use in a fixed or fluid bed contacting zone under sulfur dioxide adsorbing conditions, said particles having improved attrition resistance resulting from the above method.
Other and more particular aspects of the invention will be evident from the description below.